1. Field of the Invention
This invention relates to an imaging element and process which rely upon the photoinitiated reduction of a tetrazolium salt to form a formazan dye. For stabilized images, a subsequent or simultaneous chelation is preferred utilizing a metal ion.
2. State of the Prior Art.
Tetrazolium salts are a common source of imaging dyes, and in many instances are converted through the action of a reducing agent of some kind. British Pat. No. 670,833 is representative of patents which disclose the photoreduction of such salts. In one exemplary form the imagewise radiation of ferric ammonium salts of citric acid converts Fe(III) to Fe(II), and Fe(II) thereafter reduces the tetrazolium salt to the formazan dye. An organic hydroxy acid or its ammonium salt is disclosed as being useful in removing the undesirable brown oxide that results from this process.
Other patents disclosing photoreduction of tetrazolium salts by the use of other reducing agents are U.S. Pat. No. 3,655,383 and British Pat. No. 954,198.
To overcome the tendency to fade which characterizes such formazan dyes, metal compounds have been added to chelate the dye and to thereby color-shift it. Typical metals include metals such as cobalt, iron, nickel, etc., and typically they are added in the form of acid salts such as nitrates, and in the form of organic complexes. Examples are disclosed in U.S. Pat. No. 3,503,741 and Brit. Pat. 1,016,822.
Yet another use of the photoreduction of tetrazolium salts is disclosed in commonly assigned applications U.S. Ser. Nos. 384,858; 384,859; and 384,861; filed on Aug. 2, 1973; by D. Bailey, R. Brongo et al, and J. Fleming et al, respectively, entitled "Photographic Elements and Processes for Producing Formazan Dye Images of Enhanced Stability", "Tetrazolium Salt Photoreductive Imaging", and "Photographic Elements and Processes For Incorporated Hydrogen Source Photoreduction Imaging", respectively. Ser. No. 384,861 concerns the reduction of certain imaging means, such as tetrazolium salts, by a specific novel class of photoreductants, namely those which incorporate an internal hydrogen source. Ser. No. 384,859 is directed to the combination of tetrazolium salts and photoreductants broadly, while Ser. No. 384,858 concerns an improved class of tetrazolium salts in combination with photoreductants. Ser. No. 384,858 issued as U.S. Pat. No. 3,887,372, Ser. No. 384,859 issued as U.S. Pat. No. 3,887,374, and Ser. No. 384,861 has been refiled as U.S. Ser. No. 509,616 on Sept. 26, 1974.
Although each of these prior systems provide useful visible imaging means, they are characterized by the requirement that a substance additional to the tetrazolium salt must be added as a photoactivator. Although British Pat. No. 1,016,822, published Jan. 12, 1966, and Schiele U.S. Pat. No. 3,278,366, issued Oct. 11, 1966, teach the self-initiated reduction of a tetrazolium salt to a formazan dye, the salts involved do so only in response to radiation of 200 nm or less. It will be readily appreciated that an element which incorporates a tetrazolium salt that is itself sensitive to radiation wavelengths greater than 200 nm will be a distinct improvement.
An undesired side result of such prior systems is that blue formazan dyes, if formed, often are derived from, and therefore leave in the background areas, tetrazolium salts having an undesirable yellow color. Blue formazan dyes imagewise formed so as to leave clear background areas would represent a further improvement.
Patents pertinent only to the background of the use of certain acids as photoreductants are U.S. Pat. Nos. 2,915,392 and 3,642,478. Patents directed to methods of making certain tetrazolcarboxylic acid salts include French Pat. No. 1,099,845.